A number of methods are known for the preparation of CO- and NO-free olefin complex compounds of the transition metals. The method developed by Wilke et al., whereby transition metal salts are reduced by means of organometallic compounds of metals of Groups I to III of the Periodic Table, especially aluminum, has found the widest application. As described in German Pat. No. 1,191,375, nickel(II) acetylacetonate reacts with monoethoxydiethyl aluminum in the presence of cyclooctadiene(1,5) to form bis-(cyclooctadiene)-nickel(O), (C.sub.8 H.sub.12).sub.2 Ni(O). If, instead of cyclooctadiene, all-trans-cyclododecatriene is added, a cyclododecatriene-nickel(O), C.sub.12 H.sub.18 Ni(O) is formed, or if a cyclooctatetraene is added, a cyclooctatetraene-nickel(O), (C.sub.8 H.sub.8 Ni).sub.2, is formed.
Furthermore, it is possible to obtain such transition metal compounds by electrochemical reduction (German Offenlegungsschrift No. 2007076). The method of the co-condensation of transition metal atoms and olefins, which requires relatively extensive apparatus, also produces such complex compounds (Angew. Chem. 87, p 217, 1975). Nevertheless, they are obtainable by this method in no more than a moderate yield.
All of the above-named and hitherto known methods lead to transition metal-olefin complex compounds in which the lowest oxidation number of the transition metal atom amounts to zero.